Automobile's bodies, household electrical appliances or the like are commercialized by forming metal moldings from metal materials such as a steel sheet, a galvanized steel sheet, an aluminum alloy or the like, coating and assembling. Coating of such metal moldings are conducted after performing various steps such as degreasing, surface conditioning, chemical conversion treatment, and electrodeposition.
Surface conditioning is a treatment applied in such a way that a coat comprising phosphate crystals is formed uniformly and quickly with a high density on the whole surface of metal in chemical conversion treatment of a phosphate coat of the subsequent step, and a treatment in which crystal nuclei of phosphate are generally formed on the metal surface by immersing a metal in a surface conditioning tank.
For example, in Japanese Kokai Publication Hei-10-245685, there is disclosed a pretreatment solution for conditioning a surface before applying chemical conversion treatment of a metal phosphate coat, which contains one or more species selected from phosphate containing at least one species of bivalent or trivalent metals including fine particles having a particle diameter of 5 μm or less, alkali metal salt or ammonium salt or a mixture thereof and at least one species selected from the group of oxide particles bearing anionic charges and dispersed, an anionic water-soluble organic polymer, a nonionic water-soluble organic polymer, an anionic surfactant and a nonionic surfactant, and is adjusted to pH 4 to 13.
Further, in Japanese Kokai Publication 2000-96256, there is disclosed a treatment solution for conditioning a surface before applying chemical conversion treatment of a phosphate coat, which contains particles of one or more species of phosphate selected from phosphate containing one or more species of bivalent and/or trivalent metals and further contains (1) one or more species selected from monosaccharides, polysaccharides and derivatives thereof, or (2) one or more species of orthophosphoric acid, polyphosphoric acid or organic phosphonic acid compounds, or (3) one or more species of water-soluble high polymer compounds which consists of polymer of vinyl acetate, its derivative or copolymer of monomer being copolymerizable with vinyl acetate and vinyl acetate, or (4) polymer or copolymer obtained by polymerizing at least one species selected from specific monomers or α, β unsaturated carboxylic acid monomers, and monomer being copolymerizable with the above monomer in an amount of 50% by weight or less.
However, the treatment solution for conditioning a surface disclosed here has a problem that in an area where an aluminum alloy is in contact with a steel sheet or a galvanized steel sheet, a chemical conversion coat is hard to be formed on the aluminum alloy since a portion of an aluminum alloy becomes an anode and a portion of a steel sheet or a galvanized steel sheet becomes a cathode. Therefore, it is desired to develop a surface conditioner which can inhibit galvanic corrosion on an aluminum alloy in chemical conversion treatment.
And, these treatment solutions for conditioning a surface have a problem that a sufficient chemical conversion coat is not formed on a metal surface when applied to metals such as an aluminum alloy or a high tensile strength steel sheet. Further, these treatment solutions for conditioning a surface also have a problem that since these treatment solutions have a large particle diameter and the stability of particles in a treatment bath is insufficient, particles are apt to precipitate.